The present invention relates generally to metal ohmic contacts and, more particularly, to ohmic metal contacts with InP semiconductor compounds for use in various semiconductor devices and applications. The present invention also relates to methods of making ohmic metal contacts with InP semiconductor compounds.
In the semiconductor arts, metal contacts to semiconductor materials within a semiconductor device are commonly used. Generally speaking, these contacts fall into two types, namely ohmic contacts and rectifying contacts (also referred to as Schottky contacts). Ohmic contacts are the type of contact normally fabricated for the purpose of interconnecting devices in a semiconductor integrated circuit although non-ohmic contacts do have certain uses.
In general, when a metal to semiconductor contact is fabricated, it can either provide an ohmic contact (which means that a near linear current-voltage characteristic occurs for both directions of current flow through the contact) or the contact can be rectifying (which means that the current-voltage characteristic is decidedly non-linear as the direction of current flow reverses).
In the prior art, metal to semiconductor contacts are normally made in one of two ways. One way is to heavily dope the semiconductor material immediately adjacent the metal contact. The other technique which is commonly used is referred to as xe2x80x9calloyingxe2x80x9d. When the metal is alloyed at a predetermined temperature and for a predetermined period of time, the metal, which usually deposited on the semiconductor, penetrates into the semiconductor material. The amount of penetration of the metal into the semiconductor material generally increases with increased temperature and/or increased time.
Semiconductor materials in the prior art can either be crystalline or non-crystalline. Semiconductor devices, such as Heterostructure Field Effect Transistors (HFET), Heterojunction Bipolar Transistors (HBT), High Electron Mobility Transistors (HEMT), semiconductor light emitting diodes, semiconductor light detectors and diodes et cetera are often formed of crystalline layers of semiconductor compounds of the III-V groups. As such, one or more of the layers may comprise Indium Phosphide (InP) or an InP semiconductor compound. With respect to the devices noted above, they usually comprise a number of semiconductor layers and each layer tends to be grown upon a preceding layer. In order for crystal growth to proceed properly, it is very desirable that each layer comprise a single crystalline structure so that the following layers enjoys that same characteristic. Such crystalline layers are often referred to as being epitaxial. Generally speaking, it is desirable to minimize the amount of heating which such devices are subjected to during device manufacture.
Those skilled in the art will appreciate, of course, that sometimes a layer comprising many different crystals, which layer is then referred to as being poly-crystalline, can also be utilized.
Generally speaking, the efficiency of an ohmic contact (i.e., the amount of resistance which occurs at a metal semiconductor interface) is referred to as its specific contact resistivity. Values of specific contact resistivity acceptable in the manufacture of Integrated Circuits (ICs) typically fall in the range of 5xc3x9710xe2x88x926 to 10xe2x88x928 xcexa9-cm2.
The present invention relates to the formation of non-alloyed metal ohmic contact to InP. InP is a semiconductor compound of the III-V group which is commonly used in a wide range of modern semiconductor devices such as the so-called High Electron Mobility Transistor (HEMT) and the so-called Heterojunction Bipolar Transistor (HBT) devices. Specific contact resistivities below 5xc3x9710xe2x88x926 xcexa9-cm2 have been measured for non-alloyed Ti/Pt/Au/contacts deposited on undoped non-stoichiometric InP layers. The formation of non-stoichiometric InP layers is described in a paper entitled xe2x80x9cNew Results for Non-Stoichiometric InP Grown by Low Temperature MBExe2x80x9d delivered by one of the inventors hereof at the 10th International Conference on Indium Phosphide and Related Materials May 11-15, 1998 Tsukuba, Japan. The aforementioned paper is hereby incorporated herein by reference.
The present invention permits the fabrication of non-alloyed ohmic metal contacts to InP semiconductor compounds without the use of heavy doping levels, which levels are defined as being doping levels above 1019 cmxe2x88x923, of shallow impurities.
It is believed that the present invention may also be used to further reduce the specific contact resistance of ohmic contacts made to heavily doped InP.
The HEMT and HBT devices noted above may use InP collector and/or emitter layers (and possibly other layers as well) which can benefit from reliable ohmic contacts without the need to rely on alloy and/or heavy doping to achieve an ohmic contact.
Molecular Beam Epitaxy (MBE) can be used to grow layers of materials of the III-V compound semiconductor group. Indeed, the compound produced can be non-stoichiometric through the incorporation of excess amounts of the group V element. As has been demonstrated in the paper entitled xe2x80x9cNew Results for Non-Stoichiometric InP Growth by Low Temperature MBExe2x80x9d referred to above, non-stoichiometric InP layers remain crystalline when they contain 0.6% excess phosphorous. The results also showed that the substrate temperatures at which good crystalline layers of non-stoichiometric InP can be grown is relatively narrow, between 260xc2x0 C. and 340xc2x0 C.
Low resistance, non-alloyed ohmic contacts can be fabricated on undoped non-stoichiometric InP films which contain 0.6% excess phosphorous. It is believed that the percentage of excess phosphorous is not particularly critical and it is believed that non-stoichiometric InP films which contain as little as 0.1% excess phosphorous can also likely produce low resistance non-alloyed ohmic contacts. The upper limit of the amount of phosphorous which may be used in the non-stoichiometric InP film is not known, although it is known that when the InP film contains approximately 2% excess phosphorous, the InP film becomes poly-crystalline (the phosphorous starts to precipitate out at that concentration). Since many applications require the use of crystalline InP films, the point at which the excess phosphorous causes the film to become poly-crystalline by precipitation acts as a practical upper limit for the amount of excess phosphorous which may be utilized in such applications. However, certain applications can utilize poly-crystalline InP films and therefore the amount of excess phosphorous can be higher than 2% and it is believed that the amount of excess phosphorous can be as least as high as 11% as reported in the aforementioned paper.
The group III-V semiconductor compounds are often grown in epitaxial layers using MBE. The lattice constants of the layers should either be matched or at least approximately matched or the layers should be sufficiently,thin to that they will deform to accommodate the lattice constant of the underlying layer. Devices with such thin, deformed layers are often called xe2x80x9cpseudomorphic.xe2x80x9d Pseudomorphic layers, while thermodynamically stable for layer thicknesses below some strain-dependant level, can relax due the formation of arrays of misfit dislocations of the otherwise crystalline layer structure if subjected to undue heating in subsequent processing steps. As such, it is often desirable to limit the heating of devices made of epitaxial layers, especially when the layers are pseudomorphic. This invention helps reduce and/or eliminate heating which might be otherwise employed to achieve a metal to semiconductor ohmic contact with InP semiconductor compounds.